Hydroxy aromatic acids are useful as intermediates and additives in the manufacture of many valuable materials including pharmaceuticals and compounds active in crop protection, and are also useful as monomers in the production of polymers. Salicylic acid (o-hydroxybenzoic acid), for example, is used in the manufacture of aspirin and has other pharmaceutical applications. Esters of p-hydroxybenzoic acid, known as “parabens”, are used as food and cosmetic preservatives. P-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid are each used as a component of liquid crystalline polymers.
Various preparations of hydroxybenzoic acids, including 2,5-dihydroxyterephthalic acid (“DHTA”), are known. Marzin, in Journal fuer Praktische Chemie, 1933, 138, 103-106, teaches the synthesis of 2,5-dihydroxyterephthalic acid (“DHTA”) from 2,5-dibromoterephthalic acid (“DBTA”) in the presence of copper powder.
Singh et al, in Jour. Indian Chem. Soc., Vol. 34, No. 4, pages 321-323 (1957), report the preparation of a product that includes DHTA by the condensation of DBTA with phenol in the presence of KOH and copper powder.
Rusonik et al, Dalton Trans., 2003, 2024-2028, describe the transformation of 2-bromobenzoic acid into salicylic acid, benzoic acid, and diphenoic acid in a reaction catalyzed by Cu(I) in the presence of various ligands. A tertiary tetraamine minimizes the formation of diphenoic acid in use with Cu(I).
Comdom et al, Synthetic Communications, 32(13), 2055-59 (2002), describe a process for the synthesis of salicylic acids from 2-chlorobenzoic acids. Stoichiometric amounts of pyridine (0.5 to 2.0 moles per mole of 2-chlorobenzoic acid) are used such as at least 1.0 mole pyridine per mole 2-chlorobenzoic acid. Cu powder is used as a catalyst along with the pyridine.
Gelmont et al, Organic Process Research & Development, 6(5), 591-596 (2002), and U.S. Pat. No. 5,703,274, describe a process for the preparation of 5-hydroxyisophthalic acid by hydrolyzing 5-bromoisophthalic acid, mixtures of 5-bromoisophthalic acid, dibromoisophthalic acid isomers, and salts thereof in an aqueous alkaline solution in the presence of a copper catalyst at a temperature of 100 to 270° C.
Israeli Patent 112,706 discloses a process for the preparation of 4-hydroxyphthalic acid, and a mixture of 3- and 4-hydroxyphthalic acids, by hydrolyzing the corresponding bromophthalic acids in an aqueous alkaline solution in the presence of a copper catalyst at a temperature of 100 to 160° C. Examples of copper catalysts disclosed include Cu(0), CuCl, CuCl2, Cu2O, CuO, CuBr2, CUSO4, Cu(OH)2, and copper (II) acetate.
The various prior art processes for making hydroxybenzoic acids are characterized by long reaction times, limited conversion resulting in significant productivity loss, or the need to run under pressure and/or at higher temperatures (typically 140 to 250° C.) to get reasonable rates and productivity. A need therefore remains for a process by which hydroxybenzoic acids can be produced economically; with low inherent operational difficulty; and with high yields and high productivity in small- and large-scale operation, and in batch and continuous operation.